It said a Dutch submarine might have triggered Sweden's alert after carrying out exercises in the area.

However, this theory has been dismissed by the Dutch defence ministry.

"It was definitely not a Dutch submarine," a ministry spokeswoman in The Hague told the BBC.

'Pattern of incidents'

It is the fourth day of the search, involving ships and helicopters.

The Swedish military earlier said it was investigating three credible sightings of "foreign underwater activity". It denied looking for a submarine, and said that it was conducting an intelligence operation.

It also released a grainy photograph showing an object in the sea near Stockholm, although it was not clear what the object was.

The Swedish navy has asked all vessels to stay at least 10km away from military vessels.

A Swedish official, speaking on condition of anonymity, told the BBC that air space around the search area had been closed off.

He said he doubted the incident could have been a navigating error, as the Stockholm archipelago is a difficult area to navigate.

When asked why an underwater vessel would have entered waters off Sweden, the official said: "It's always about testing the Swedish response to see if we are awake and able to detect it... it's a way of testing our skills in detecting and locating."

Sweden was keeping an open mind about which country was behind the incident, the official said, adding that there had been a similar pattern of incidents in the past.

Soviet submarine sightings during the Cold War caused security alerts in Sweden in the 1980s.

Swedish radio says the search area has expanded to include more islands out into the Baltic, further away from Stockholm.

Swedish news website The Local says vessels and troops have been deployed from Moeja, north of Stockholm, to Ornoe, south of the capital, using sensors in an operation "continuing day and night".

A local newspaper said Sweden had intercepted a distress signal in Russian.

Image copyrightEPA/HANDOUTImage caption
The Swedish military handed out an image taken by a passerby showing an object in the sea near Stockholm

It is reported to be Sweden's biggest military mobilisation since the Cold War, the BBC's World Affairs Correspondent Richard Galpin reports.

Whatever is lurking under the waters of the Stockholm archipelago, it is raising tensions in what is already a nervous region after Russian troops moved into parts of Ukraine earlier this year, our correspondent adds.

Russia has several submarines based in Kaliningrad, a Russian enclave bordered by Poland and Lithuania and facing out to Sweden, as well as a much bigger force near Murmansk on the Kola Peninsula.

Last month, another Swedish newspaper, Expressen, reported an incursion by two Russian Sukhoi Su-24 fighter bombers into national airspace. Sweden scrambled jets to see them off, it said.